JP7770175B2 - Carboxylic acid polymers with upper critical solution temperature-type temperature responsiveness and their applications - Google Patents

Description

The present invention relates to a carboxylic acid polymer with upper critical solution temperature (UCST)-type temperature responsiveness, which is useful as a driving solution for forward osmosis membrane water treatment systems and osmotic power generation, and to a forward osmosis membrane water treatment system and an osmotic power generation driving solution containing the temperature-responsive carboxylic acid polymer.

Stimuli-responsive polymers are materials whose physical and chemical properties change in response to external stimuli such as temperature, pH, ionic strength, light irradiation, and the application of electromagnetic fields. Temperature-responsive polymers in particular are being widely studied due to their potential applications in fields such as drug delivery systems, gene therapy, separation of metals and cells, bioimaging, catheters, artificial muscles, optical devices, catalysts, forward osmosis water treatment systems, and osmotic power generation (see, for example, Patent Document 1 and Patent Document 2).

Temperature-responsive polymers are polymers whose solubility in a solvent changes in response to an external temperature stimulus. Due to the temperature-induced phase change, they are classified into two types: LCST behavior, which has a lower critical solution temperature (LCST), and UCST behavior, which has an upper critical solution temperature (UCST).

LCST polymers, typified by poly(N-isopropylacrylamide) (P-NIPAM), have been actively researched, primarily in the biomedical field, and many other LCST polymers have been reported, including P-NIPAM, poly(N-isopropylmethacrylamide), poly(N-vinylcaprolactam), and poly(oligoethylene glycol) acrylate. However, there have only been a few reported examples of polymers that exhibit UCST behavior, and in particular, there have been almost no reports of ionic polymers that exhibit UCST behavior due to intermolecular interactions over a wide range of polymer concentrations.

Known ionic polymers include copolymers of acrylic acid and acrylonitrile (e.g., Non-Patent Document 1), copolymers of acrylic acid and styrene, which has excellent hydrolysis resistance (e.g., Non-Patent Document 2), and the homopolymer polyacrylic acid (PAA) and its sodium salt (PAA-Na) (e.g., Non-Patent Document 3). However, of these, only acrylic acid/acrylonitrile copolymers have been reported as temperature-responsive polymers.

The inventors have discovered that polyampholites composed of the anionic monomer styrene sulfonic acid and the cationic monomer 4-vinylbenzyltrimethylammonium exhibit UCST due to electrostatic and hydrophobic interactions between polymer molecules, and also exhibit high osmotic pressure and hydrolysis resistance, making them useful as work solutions in forward osmosis membrane water treatment systems (see, for example, Patent Document 3).

It is also known that polymers containing ester structural units in which the carboxyl group is aminoalkylated and structural units containing sulfo groups, as well as polymers further containing carboxyl group-containing structural units, can be used in the work solution of the above-mentioned forward osmosis membrane water treatment system (e.g., Patent Document 4).

Patent No. 4069221 Patent No. 6125863 International Publication No. 2020/188839 Japanese Patent Application Laid-Open No. 2021-107491

Chuanzhuang Zhao et al.; Macromolecules, Vol. 52, pp. 4441-4446, 2019 S. Toppet et al., Journal of Polymer Science Part-A, Vol. 13, No. 8, pp. 1879-1887, 1975 Qingchun Ge et al.; Water Research, Vol. 46, pp. 1318-1326, 2012

Non-Patent Document 1 describes that a 0.5 wt % to 2.0 wt % aqueous solution of an acrylic acid copolymer containing 4.5 mol % to 22 mol % of acrylonitrile polymerized units and having a number average molecular weight of 27,000 to 37,000 daltons exhibits UCST properties of 7.2°C to 37.9°C.
However, there was no mention of physical properties in the high concentration range, and there was a problem with the manifestation of UCST behavior over a wide polymer concentration range. Since the molecular weight exceeds 20,000, in this case, the viscosity and UCST in the high concentration range are expected to be quite high. In addition, since the cyano group of the acrylonitrile polymerization unit is hydrolyzed to a carboxyl group in high-temperature water, there is a problem in manifesting the effect stably and reproducibly, as the UCST decreases or disappears over time.

Non-Patent Document 2 reports on the copolymerization of acrylic acid and styrene, but does not mention the relationship between polymer structure and temperature responsiveness, such as UCST. The document also reports that the copolymerization of monomers with functional groups capable of forming hydrogen bonds with solvents, such as the carboxyl group of acrylic acid, is significantly affected by factors such as the reaction solvent on the reactivity ratio. Until now, it has been difficult to even design molecules capable of producing UCST as an anionic polymer.

Non-Patent Document 3 indicates that the homopolymer polyacrylic acid sodium salt (PAA-Na) has potential as a working solution for forward osmosis membrane water treatment systems, but there is no description of a carboxylic acid-based copolymer with temperature responsiveness, and it remains difficult to design specific molecules capable of exhibiting UCST.

Conventional upper critical solution temperature-responsive polymers (hereinafter sometimes referred to as UCST-type polymers) have issues, such as using expensive cationic monomers as raw materials and having excessively high UCST and aqueous solution viscosity. Therefore, there has been a demand for low-cost UCST-type polymers with low UCST and aqueous solution viscosity.

The issues with conventional UCST-type polymers are explained below using the draw solution (hereinafter sometimes referred to as DS, an abbreviation for Draw Solution) in a forward osmosis membrane water treatment system as an example of its application.

Figure 1 is a diagram showing a phase separation curve of a UCST polymer. The upper part of the solid line in Figure 1 represents the homogeneous phase in which the polymer is dissolved, and the lower part represents the two-phase separation state. Figure 2 shows a schematic diagram of a forward osmosis water treatment system using a UCST (cooling separation) working solution (DS). In Figures 1 and 2, a to e correspond to each other in Figures 1 and 2.
For example, the steps a → b, b → c → d/e, and d → a will be described below.
Step a → b: A 40 wt % aqueous polymer solution having sufficient osmotic pressure as a draw solution (DS), shown as a in Figures 1 and 2, draws water from the water to be treated at 50°C through a semipermeable membrane, and as shown as b in Figures 1 and 2, is diluted to a concentration of 20 wt % and its osmotic pressure (water absorption capacity) decreases.
Steps b → c → d/e: When the diluted 20 wt % aqueous polymer solution b is cooled to 20°C as shown by c in Figures 1 and 2, phase separation occurs, and fresh water shown by e in Figures 1 and 2 and a 40 wt % aqueous polymer solution shown by d in Figures 1 and 2 are reproduced.
Step d→step a: The regenerated 40 wt % aqueous polymer solution is heated to 50° C., and fresh water is again sucked out from the water to be treated.
The above-described series of cycles is shown schematically in FIG. 2, and the relationship between the polymer concentration and the phase separation temperature in each step is shown in FIG.

The ideal phase separation curve slope and phase separation temperature for the above system cannot be generalized, as they vary depending on the temperature of the water to be treated, the operating temperature, the osmotic pressure and viscosity of the work solution, and the physical properties of the semipermeable membrane used. For example, with the phase separation curve slope shown in Figure 1, approximately the same amount of fresh water as the work solution can be produced per cycle. In contrast, if the phase separation curve slope is twice as large as in Figure 1, the amount of fresh water produced per cycle will be halved. Conversely, if the slope is halved, the amount of fresh water produced per cycle will increase.

Furthermore, if the phase separation temperature (dissolution temperature) is too high, it becomes difficult to produce freshwater at around 40°C to 60°C from warm seawater or petroleum-produced water. Furthermore, if the viscosity of the aqueous solution is too high, a burden is placed on the circulation of the working solution, especially after it has been cooled/regenerated, and the water absorption rate decreases due to concentration polarization within the semipermeable membrane (making it difficult for the polymer to diffuse into the support layer).

The polyampholite described in Patent Document 3 (WO 2020/188839) had problems such as: (1) a dramatic increase in UCST in the absence of an inorganic salt, and an excessively steep slope of the phase separation curve; (2) an excessively high viscosity of the aqueous solution; and (3) high costs due to the use of a large amount of expensive cationic monomers.
The polymer described in Patent Document 4 (JP 2021-107491 A) is a strong electrolyte polymer in which amino groups and sulfo groups are essential in the polymer side chains, and furthermore, the above-mentioned ester structural unit is used as the skeleton, so there was a concern about hydrolysis.
As described in Patent Documents 3 and 4, copolymerization of a monomer having a functional group capable of forming a hydrogen bond with a solvent, such as the carboxyl group of acrylic acid, has traditionally been difficult, even in terms of molecular design that can express UCST as an anionic polymer.
Furthermore, conventional UCST-type polymers (e.g., Patent Documents 3 and 4) are strong electrolyte polymers, and although they have the advantage of high osmotic pressure, in the absence of inorganic salts they have problems such as an excessively steep slope of the phase separation curve, an excessively high phase separation temperature, and an excessively high aqueous solution viscosity, and therefore there has been a strong demand for improvements.

Therefore, an object of the present invention is to provide a material that can solve the above-mentioned conventional problems and a system that can utilize the material.
Specifically, the present invention provides a carboxylic acid polymer having upper critical solution temperature (UCST)-type temperature responsiveness, which is useful as a driving solution for a forward osmosis membrane water treatment system or an osmotic pressure power generation system, and a driving solution for a forward osmosis membrane water treatment system or an osmotic pressure power generation system, which contains a temperature-responsive carboxylic acid polymer for such an application.

After extensive research, the inventors discovered that polymers composed primarily of versatile, inexpensive carboxyl-containing vinyl monomers such as acrylic acid or methacrylic acid, copolymerized with specific amounts of aromatic vinyl monomers such as styrene or vinyl benzoic acid, have low solution viscosity within a specific range of molecular weight and molecular weight distribution, and exhibit UCST-type phase separation properties that make them suitable for use as a driving solution in forward osmosis membrane water treatment systems and osmotic power generation, leading to the completion of the present invention.

That is, the present invention relates to the following inventions.
[1]
A temperature-responsive carboxylic acid polymer comprising the following structural units (A) and (B), wherein the content of the structural unit (B) is 5 mol % to 30 mol % based on the total of the structural units (A) and (B), the polymer has a number average molecular weight Mn of 500 to 20,000 daltons (Da), and a ratio Mw/Mn of the number average molecular weight Mn to the weight average molecular weight Mw of 1.00 to 2.00; the polymer is water-soluble and has an upper critical solution temperature (hereinafter, sometimes abbreviated as UCST), and the viscosity of a 50 wt % aqueous solution thereof at 25°C is 10 mPa·s to 10,000 mPa·s, and the polymer is hydrophobic at temperatures below the UCST and hydrophilic at temperatures equal to or higher than the UCST.
The structural unit (A) is represented by the following general formula (1):
(In formula (1), _R1 represents a hydrogen atom or a carboxyl group, _R2 represents a hydrogen atom, a methyl group, an ethyl group or a carboxymethyl group, and M represents a proton, an ammonium cation or an alkali metal cation),
The structural unit (B) is represented by the following general formula (2):
(In formula (2), _R3 represents a hydrogen atom, a hydroxyl group, or a carboxyl group.)
[2]
Item [1] The carboxylic acid polymer according to item [1], further comprising the following structural unit (C) in addition to the structural units (A) and (B), and the content of the structural unit (C) is 0.1 mol % to 10 mol % based on the total of the structural units (A) to (C).
The structural unit (C) is represented by the following general formula (3):
(In formula (3), Q represents a structural unit derived from an acrylamide vinyl monomer.)
[3]
The carboxylic acid polymer according to item [1] or [2], wherein the number average molecular weight Mn is 500 to 5,000 Daltons (Da).
[4]
The carboxylic acid polymer according to item [1], wherein the structural unit (A) is a structural unit derived from at least one carboxyl group-containing vinyl monomer selected from the group consisting of acrylic acid, methacrylic acid, ethacrylic acid, itaconic acid, and maleic acid, and the structural unit (B) is a structural unit derived from at least one aromatic vinyl monomer selected from the group consisting of styrene, vinylbenzoic acid, and hydroxystyrene.
[5]
Item [2]. The carboxylic acid polymer according to item [2], wherein the structural unit (A) is a structural unit derived from at least one vinyl monomer selected from the group consisting of acrylic acid, methacrylic acid, ethacrylic acid, itaconic acid, and maleic acid; the structural unit (B) is a structural unit derived from at least one vinyl monomer selected from the group consisting of styrene, vinylbenzoic acid, and hydroxystyrene; and the structural unit (C) is a structural unit derived from at least one acrylamide-based vinyl monomer selected from the group consisting of acrylamide, methacrylamide, 4-acroylmorpholine, and 4-methacryloylmorpholine.
[6]
[5] The carboxylic acid polymer according to any one of [1] to [5], wherein the UCST of a 20 wt% aqueous solution is 50°C to 100°C, that of a 30 wt% aqueous solution is 40°C to 90°C, that of a 40 wt% aqueous solution is 30°C to 70°C, and that of a 50 wt% aqueous solution is 10°C to 60°C.
[7]
[6] The carboxylic acid polymer according to any one of [1] to [6], wherein the viscosity of a 50 wt% aqueous solution at 40°C is 10 mPa·s to 5000 mPa·s.
[8]
Item [1] to [7], wherein the polymer has a structure in which the terminal of the polymer is a carboxyl group.
[9]
A method for producing a temperature-responsive carboxylic acid polymer that is hydrophobic below UCST and hydrophilic at or above UCST, comprising polymerizing a monomer mixture containing a carboxyl group-containing vinyl monomer and an aromatic vinyl monomer in a hydrophilic solvent using a hydrophilic azo radical polymerization initiator, or a hydrophilic azo radical polymerization initiator and a RAFT agent or a hydrophilic chain transfer agent,
the ratio of the aromatic vinyl monomer to all monomers in the monomer mixture is 9.0 mol % to 20.0 mol %;
the total ratio of the hydrophilic azo radical polymerization initiator, the RAFT agent, and the hydrophilic chain transfer agent to all the monomers in the monomer mixture is 0.90 mol % to 13.0 mol %;
method.
[10]
the carboxyl group-containing vinyl monomer is one or more carboxyl group-containing vinyl monomers selected from the group consisting of acrylic acid, methacrylic acid, ethacrylic acid, itaconic acid, and maleic acid;
The aromatic vinyl monomer is one or more aromatic vinyl monomers selected from the group consisting of styrene, vinyl benzoic acid, and hydroxystyrene;
The manufacturing method according to item [9].
[11]
Item [9] or [10], the monomer mixture further contains an acrylamide vinyl monomer, the manufacturing method according to item [9] or [10],
the ratio of the aromatic vinyl monomer to all the monomers in the monomer mixture is 10.0 mol% to 15.0 mol%, and the ratio of the acrylamide vinyl monomer is 0.1 mol% to 5.0 mol%;
method.
[12]
Item [11] The method according to item [11], wherein the acrylamide vinyl monomer is one or more acrylamide vinyl monomers selected from the group consisting of acrylamide, methacrylamide, 4-acroylmorpholine, and 4-methacryloylmorpholine.
[13]
Item [9] to [12]. The method according to any one of items [9] to [12], wherein the hydrophilic azo radical polymerization initiator is a hydrophilic polymerization initiator having a carboxyl group, the RAFT agent is a RAFT agent having a carboxyl group, and the hydrophilic chain transfer agent is a hydrophilic chain transfer agent having a carboxyl group.
[14]
The method according to any one of items [9] to [13], wherein the hydrophilic solvent is one or more selected from the group consisting of methanol, ethanol, propanol, butanol, acetone, and water.
[15]
An aqueous solution containing 1% by weight to 70% by weight of the carboxylic acid polymer according to any one of items [1] to [8], which undergoes phase separation below the UCST and becomes a homogeneous solution above the UCST.
[16]
Item [15] The aqueous solution according to item [15], wherein the content of the carboxylic acid polymer is 20% by weight to 50% by weight.
[17]
Item [15] or [16], wherein the phase separation temperature is 40 ° C to 60 ° C. The aqueous solution according to item [16] or [16].
[18]
A work solution for a forward osmosis membrane water treatment system or an osmotic pressure power generation system, comprising the carboxylic acid polymer according to any one of items [1] to [8] as a solute.

The UCST-type temperature-responsive polymer of the present invention is a water-soluble carboxylic acid-based polymer whose main component is a versatile, inexpensive carboxyl-group-containing vinyl monomer, such as acrylic acid or methacrylic acid, copolymerized with a predetermined amount of an aromatic vinyl monomer, such as styrene or vinyl benzoic acid. It also has a predetermined molecular weight and molecular weight distribution. Therefore, compared to conventional ionic UCST-type polymers, it exhibits a low UCST and a phase separation curve with a small slope, despite containing no inorganic salts or only small amounts of inorganic salts. Furthermore, because the viscosity of the aqueous solution is extremely low, it can be used as a driving solution for forward osmosis membrane water treatment systems and osmotic power generation.

1 is a diagram showing a schematic diagram of a phase separation curve of a UCST-type polymer, in which the horizontal axis indicates the concentration of the polymer aqueous solution (unit: wt % (weight %)) and the vertical axis indicates the phase separation temperature (unit: ° C.). A to E in FIG. 1 correspond to A to E in FIG. 2, which shows an embodiment of a forward osmosis membrane water treatment system. A schematic diagram of a forward osmosis membrane water treatment system using a UCST-type (cooling separation type) working solution (DS) is shown in Figure 2. A to E in Figure 2 correspond to A to E in Figure 1, which shows the phase separation curve of the UCST-type polymer. 1 shows the ¹³ C-NMR spectrum of the acrylic acid/styrene copolymer (after first purification) obtained in Example 6, where the horizontal axis indicates chemical shift (unit: ppm) and the vertical axis indicates relative signal intensity. This is an enlarged view of the high magnetic field side of the ¹³ C-NMR spectrum of FIG. 3 (see Table 1 for the assignment of each peak). This is an enlarged view of the low magnetic field side of the ¹³ C-NMR spectrum of FIG. 3 (see Table 1 for the assignment of each peak).

The following describes in detail the mode for carrying out the present invention (hereinafter referred to as the "present embodiment"). Note that the present invention is not limited to the following present embodiment. The present invention can be modified and implemented as appropriate within the scope of its gist.

The carboxyl group-containing monomer used in the present invention is the main component of the UCST polymer of the present invention and is a water-soluble monomer necessary for forming the structural unit (A) represented by the above general formula (1).

Carboxyl group-containing monomers used in the present invention include acrylic acid, methacrylic acid, ethacrylic acid, maleic acid, and itaconic acid. Of these, acrylic acid and methacrylic acid are preferred from the viewpoint of polymerizability, and acrylic acid is more preferred from the viewpoint of the osmotic pressure of the polymer. For the carboxyl groups of the structural unit (A) contained in the carboxylic acid polymer of the present invention, alkali neutralization is advantageous from the viewpoint of osmotic pressure. However, since water solubility changes sharply depending on the degree of neutralization, making it difficult to control the UCST, the degree of neutralization (alkali equivalent to the carboxyl group) is basically sufficient at 0%. However, depending on the type and content of the structural unit (B), an excessively high UCST can be reduced by neutralization, so the degree of neutralization should be greater than 0% and up to 10%, and preferably 0.01% to 10%.

The aromatic vinyl monomer used in the present invention is a hydrophobic monomer necessary for forming the structural unit (B) represented by the above general formula (2) contained in the carboxylic acid polymer of the present invention, and is not particularly limited. Examples include styrene, chlorostyrene, bromostyrene, fluorostyrene, α-methylstyrene, cyanostyrene, methoxystyrene, para-t-butoxystyrene, para-acetoxystyrene, para-1-ethoxyethoxystyrene, vinylnaphthalene, vinylbenzoic acid, and styrenephosphonic acid. Of these, styrene is preferred from the perspective of cost, and vinylbenzoic acid and styrenephosphonic acid are preferred from the perspective of a wide range of control over phase separation.

The monomer necessary to form the structural unit (C) used in the present invention is not particularly limited as long as it can be copolymerized with the above-mentioned carboxyl group-containing vinyl monomer and aromatic vinyl monomer. Examples include acrylamide, methacrylamide, acroylmorpholine, methacryloylmorpholine, N-phenylmaleimide, N-cyclohexylmaleimide, acrylonitrile, vinyl chloride, etc. However, from the viewpoint of copolymerizability and phase separation, hydrophilic acrylamide-based monomers such as acrylamide, methacrylamide, acroylmorpholine, and methacryloylmorpholine are preferred.

The UCST-type phase separation behavior exhibited by the polymer of the present invention is believed to be primarily due to hydrophobic interactions resulting from aromatic rings in the polymer. For this reason, the molar ratio of the structural units (A) and (B) contained in the polymer is extremely important, and the content of structural unit (B) relative to the total of structural units (A) and (B) is preferably 5.00 mol% to 30.00 mol%. To control the phase separation temperature within a more appropriate range, a ratio of 5.00 mol% to 20.00 mol% is preferred, and 5.00 mol% to 15.00 mol% is even more preferred.

The monomer that forms structural unit (C) is used for the purpose of adjusting the phase separation temperature, aqueous solution viscosity, etc., within a range that does not impair the durability or osmotic pressure of the polymer composed of structural units (A) and (B). Therefore, the content of structural unit (C) is preferably 0.10 mol % to 10.00 mol %, and more preferably 1.00 mol % to 5.00 mol %, of the total of structural units (A), (B), and (C).

As described above, in the present invention, the molar ratio of the structural unit (A) derived from a carboxyl group-containing vinyl monomer to the structural unit (B) derived from an aromatic vinyl monomer contained in the UCST polymer is the most important factor affecting temperature responsiveness. Furthermore, the molecular weight of the polymer is an important factor affecting the concentration polarization and viscosity within the semipermeable membrane, which affect the phase separation temperature, osmotic pressure, and water permeability. Furthermore, as described below, the terminal structure of the polymer also affects phase separation, so a structure having a carboxyl group at at least one of the polymer terminals is preferred.

The number-average molecular weight and weight-average molecular weight of the UCST polymer of the present invention can be measured by gel permeation chromatography (GPC). The number-average molecular weight is preferably 500 to 20,000 daltons (Da). If the number-average molecular weight is less than 500 daltons, the polymer may dissolve in water even at low temperatures. On the other hand, if the number-average molecular weight exceeds 20,000 daltons, the polymer may not dissolve in water even when heated. Furthermore, considering the phase separation property and whether the viscosity of the aqueous solution becomes too high, resulting in poor operability, the number-average molecular weight is preferably 800 to 13,000 daltons, and more preferably 1,000 to 5,000 daltons.
Furthermore, the narrower the molecular weight distribution, the better (sharper) the temperature responsiveness and phase separation properties become, so the smaller the value obtained by dividing the weight average molecular weight by the number average molecular weight (Mw/Mn) the better. There is no problem if it is in the range of 1.00 to 2.00, but 1.00 to 1.50 is preferable, and 1.00 to 1.20 is more preferable.

Furthermore, even if the molecular weight measured by GPC is the same, the viscosity of an aqueous solution containing the polymer varies depending on the degree of neutralization. While a lower viscosity of the aqueous solution is preferable, since the polymer of the present invention is an interactive polymer, a certain degree of viscosity increase or high viscosity is unavoidable. Viscosity can be measured using a Brookfield viscometer. The viscosity of a 50 wt% aqueous polymer solution may be 10 mPa·s to 10,000 mPa·s at 25°C, but is preferably 10 mPa·s to 10,000 mPa·s at 25°C and 10 mPa·s to 5,000 mPa·s at 40°C and 60°C, and more preferably 100 mPa·s to 6,000 mPa·s at 25°C and 30 mPa·s to 2,000 mPa·s at 40°C and 60°C.

As described above, the phase separation property of the UCST polymer of the present invention is controlled by the copolymer composition, molecular weight, and molecular weight distribution, but in consideration of use as a driving solution in a forward osmosis membrane water treatment system or osmotic pressure power generation, it is preferable that the UCST of a 20 wt % aqueous polymer solution is in the range of 50° C. to 100° C., the UCST of a 30 wt % aqueous polymer solution is in the range of 40° C. to 90° C., the UCST of a 40 wt % aqueous polymer solution is in the range of 30° C. to 70° C., and the UCST of a 50 wt % aqueous polymer solution is in the range of 10° C. to 60° C. If the temperatures deviate from these ranges, it may be difficult to efficiently obtain fresh water in the practical temperature range of 40° C. to 60° C.
The principle of osmotic power generation is similar to that of freshwater production. For example, electricity is generated by the difference in osmotic pressure between freshwater (low osmotic pressure) and a working solution (high osmotic pressure) heated using factory waste heat or solar heat, and the diluted working solution is cooled and reused.

The present invention also includes an aqueous solution containing 1% to 70% by weight of the above-mentioned UCST-type temperature-responsive carboxylic acid polymer. The polymer concentration is preferably 1% to 70% by weight, more preferably 10% to 70% by weight, and even more preferably 20% to 50% by weight. Of course, when actually circulating and absorbing water as a driving solution for forward osmosis membrane water treatment systems or osmotic power generation, a polymer concentration of 20% to 70% by weight is preferred due to osmotic pressure.

There are no particular restrictions on the phase separation temperature of an aqueous solution containing a UCST-type temperature-responsive carboxylic acid polymer. From the perspective of operational efficiency and temperature control when actually implementing a forward osmosis membrane water treatment system or osmotic power generation, a temperature of 10°C to 100°C is preferred, 30°C to 70°C is more preferred, and 40°C to 60°C is even more preferred.

The upper critical solution type temperature-responsive polymer of this embodiment can be produced by producing a copolymer by radical polymerization or anionic polymerization, and then removing low-molecular-weight components derived from unreacted monomers, initiators, molecular weight modifiers, etc. by a method such as vacuum distillation, phase separation utilizing the UCST property of the polymer, or ultrafiltration.
When using a carboxyl group-containing vinyl monomer such as acrylic acid or methacrylic acid, traditional radical polymerization or living radical polymerization is preferred. When copolymerizing an acrylic acid ester or a methacrylic acid ester and then hydrolyzing it to regenerate the carboxyl group, not only radical polymerization but also anionic polymerization can be applied. Among radical polymerizations, living radical polymerization methods such as atom transfer polymerization, reversible addition-fragmentation transfer polymerization, iodine transfer polymerization, and stable nitroxyl-mediated polymerization are more preferred from the viewpoint of obtaining polymers with narrow molecular weight distributions, and known methods can be applied (e.g., Yamako et al., Journal of the Society of Rubber Science and Technology of Japan, Vol. 82, No. 8, pp. 363-369, 2009; Uegai et al., Network Polymer, Vol. 30, No. 5, pp. 234-249, 2009).

Among the above-mentioned living radical polymerization methods, reversible addition-fragmentation transfer polymerization, so-called RAFT polymerization, will be described in detail as an example.
For example, a monomer solution in which a carboxyl group-containing vinyl monomer such as acrylic acid or methacrylic acid, an aromatic vinyl monomer such as styrene or vinylbenzoic acid, and, if necessary, a monomer copolymerizable with these monomers such as acrylamide are dissolved, a radical polymerization initiator, and a polymerization regulator such as a dithioester compound (a so-called RAFT agent used in reversible addition-fragmentation transfer polymerization) are charged into a reaction vessel, and polymerization is carried out in an inert gas atmosphere at 40°C to 100°C for 5 hours to 48 hours, thereby obtaining the carboxylic acid polymer of the present invention.

In a monomer solution containing a carboxyl group-containing vinyl monomer, an aromatic vinyl monomer, and, if necessary, a monomer copolymerizable with these, such as acrylamide, the ratio of the aromatic vinyl monomer to the total monomers in the monomer solution is preferably 5.00 mol% to 30.00 mol%, and more preferably 9.00 mol% to 20.00 mol%.

The amount of the RAFT agent used may be adjusted depending on the molecular weight of the target polymer, and is usually 0.01 mol % to 100.00 mol % relative to the number of moles of all monomers. However, in consideration of practicality, the amount is preferably 0.10 mol % to 30.00 mol %, and more preferably 0.50 mol % to 5.00 mol %.
The amount of radical polymerization initiator used may usually be 0.01 mol % to 100.00 mol % based on the number of moles of all monomers, but in consideration of molecular weight controllability, it is preferably 0.01 mol % to 20.00 mol %, more preferably 0.10 mol % to 15.00 mol %.
As the RAFT agent, known compounds commercially available from Wako Pure Chemical Industries, Ltd., Tokyo Chemical Industry Co., Ltd., Boron Molecular, Inc., etc. can be used. For example, RAFT agents that can be used include dithioester compounds such as 4-cyano-4-(dodecylsulfanylthiocarbonyl)sulfanylpentanoic acid, 3-((((1-carboxyethyl)thio)carbonothioyl)thio)propanoic acid, 4-cyano-4-(thiobenzoylthio)pentanoic acid, 2-cyanopropan-2-yl benzodithioate, 4-cyano-4-[(thiobenzoyl)sulfanyl]pentanoic acid, 2-methyl-2-[(dodecylsulfanylthiocarbonyl)sulfanyl]propanoic acid, benzyl dithiobenzoate, 2-cyanoprop-2-yldithiobenzoate, S,S-dibenzyl trithiocarbonate, cyanomethyl(3,5-dimethyl-1H-pyrazole)carbodithioate, and cyanomethyl N-methyl-N-phenyldithiocarbamate.
Among these, from the viewpoint of the phase separation properties of the carboxylic acid polymer of the present invention, preferred RAFT agents are dithioester compounds having a carboxyl group, such as 4-cyano-4-(dodecylsulfanylthiocarbonyl)sulfanylpentanoic acid, 3-((((1-carboxyethyl)thio)carbonothioyl)thio)propanoic acid, 4-cyano-4-(thiobenzoylthio)pentanoic acid, 2-cyanopropan-2-yl benzodithioate, 4-cyano-4-[(thiobenzoyl)sulfanyl]pentanoic acid, and 2-methyl-2-[(dodecylsulfanylthiocarbonyl)sulfanyl]propanoic acid.

Examples of the radical polymerization initiator include di-t-butyl peroxide, dicumyl peroxide, t-butylcumyl peroxide, benzoyl peroxide, dilauryl peroxide, cumene hydroperoxide, t-butyl hydroperoxide, 1,1-bis(t-butylperoxy)-3,5,5-trimethylcyclohexane, 1,1-bis(t-butylperoxy)-cyclohexane, cyclohexanone peroxide, t-butylperoxybenzoate, t-butylperoxyisobutyrate, t-butylperoxy-3,5,5-trimethylhexanoate, t-butylperoxy-2-ethylhexanoate, and t-butylperoxyisopropyl. Peroxide compounds such as propyl carbonate, cumyl peroxyoctoate, potassium persulfate, ammonium persulfate, and hydrogen peroxide, 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(2-methylpropionitrile), 2,2'-azobis(2-methylbutyronitrile), 1,1'-azobis(cyclohexane-1-carbonitrile), 1-[(1-cyano-1-methylethyl)azo]formamide, dimethyl 2,2'-azobis(2-methylpropionate), 4,4'-azobis(4-cyanovaleric acid), 2,2'-azobis(2,4,4-trimethyl pentane), 2,2'-azobis{2-methyl-N-[1,1'-bis(hydroxymethyl)-2-hydroxyethyl]propionamide}, 2,2'-azobis{2-(2-imidazolin-2-yl)propane]dihydrochloride, 2,2'-azobis{2-(2-imidazolin-2-yl)propane]disulfate dihydrate, 2,2'-azobis{2-[1-(2-hydroxyethyl)-2-imidazolin-2-yl)propane]}dihydrochloride, 2,2'-azobis(1-imino-1-pyrrolidino-2-methylpropane)dihydrochloride, 2,2'-azobis(2-methylpropionamidine)dihydrochloride, 2,2'-azobis azo compounds such as bis[N-(2-carboxyethyl)-2-methylpropionamidine]tetrahydrate, 1,1'-azobis(1-acetoxy-1-phenylmethane), and 4,4'-diazenediylbis(4-cyanopentanoic acid)-α-hydro-ω-hydroxypoly(oxyethylene) polycondensate; and stable nitroxyl radicals such as N-tert-butyl-O-[1-[4-(chloromethyl)phenyl]ethyl]-N-(2-methyl-1-phenylpropyl)hydroxylamine, 2,2,6-tetramethylpiperidine 1-oxyl, and N-tert-butyl-N-(2-methyl-1-phenylpropyl)-O-(1-phenylethyl)hydroxylamine.
Among these, when a RAFT agent is used, from the viewpoint of molecular weight controllability, 2,2'-azobis{2-methyl-N-[1,1'-bis(hydroxymethyl)-2-hydroxyethyl]propionamide}, 2,2'-azobis{2-(2-imidazolin-2-yl)propane]dihydrochloride, 2,2'-azobis{2-(2-imidazolin-2-yl)propane]disulfate dihydrate, 2,2'-azobis{2-[1-(2-hydroxyethyl)-2-imidazoline-2 azo-based compounds such as 2,2'-azobis(2-methylpropionamidine) dihydrochloride, 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine]tetrahydrate, 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(isobutyronitrile), and 4,4'-azobis(4-cyanovaleric acid); From the viewpoint of phase separation, 2,2'-azobis{2-methyl-N-[1,1'-bis(hydroxymethyl)-2-hydroxyethyl]propionamide}, 2,2'-azobis{2-(2-imidazolin-2-yl)propane]dihydrochloride, 2,2'-azobis{2-(2-imidazolin-2-yl)propane]disulfate dihydrate, 2,2'-azobis{2-[1-(2-hydroxyethyl)-2-imidazolin-2-yl)propane} [N-(2-carboxyethyl)-2-methylpropionamidine]} dihydrochloride, 2,2'-azobis(2-methylpropionamidine) dihydrochloride, 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine] tetrahydrate, 4,4'-azobis(4-cyanovaleric acid), and other hydrophilic azo radical polymerization initiators are more preferred, and hydrophilic azo radical polymerization initiators containing a carboxyl group, such as 4,4'-azobis(4-cyanovaleric acid), are even more preferred.

The solvent used for polymerization is not particularly limited as long as it can uniformly dissolve the monomer mixture. Examples include alcohols such as methanol, ethanol, propanol, and butanol; cellosolves such as methoxyethanol and ethoxyethanol; hydrophilic solvents such as acetonitrile, acetone, tetrahydrofuran, dioxane, N-methylpyrrolidone, dimethylformamide, dimethyl sulfoxide, and dimethylacetamide; and mixtures of these with water; hydrocarbon solvents such as benzene, toluene, xylene, cyclohexane, methylcyclohexane, and heptane; and halogenated solvents such as chloroform and tetrachloromethane. Among these, considering the polymerization rate and the solubility of the polymerization initiator and chain transfer agent, hydrophilic solvents such as methanol, ethanol, propanol, butanol, acetone, and mixtures of these with water are preferred.

To increase the polymerization rate and conversion rate, it is preferable to have as high a monomer concentration as possible. However, taking into consideration molecular weight controllability, a concentration of 5.0% to 50.0% by weight is preferred. Taking into consideration reaction temperature control, a concentration of 5.0% to 30.0% by weight is more preferred, and 9.0% to 20.0% by weight is even more preferred.

Since the polymerization solution may contain unreacted monomers, impurities derived from the polymerization initiator and RAFT agent, and oligomers with a molecular weight lower than the target molecular weight, it is preferable to purify it by utilizing the UCST properties of the polymer. For example, the polymer is heated and dissolved in water at a low concentration, and then cooled to separate into two layers: a concentrated aqueous polymer solution and a supernatant containing low-molecular-weight impurities. The concentrated aqueous polymer solution is then recovered. Repeating this process can improve the purity of the polymer and the monodispersity of its molecular weight. Alternatively, impurities in the polymer solution can be removed using an ultrafiltration membrane.

In addition to living radical polymerization typified by the above-mentioned RAFT polymerization, traditional radical polymerization can also be applied to the production method of the carboxylic acid polymer of the present invention. Examples include a batch addition polymerization method in which a monomer solution containing a carboxyl group-containing vinyl monomer such as acrylic acid or methacrylic acid, an aromatic vinyl monomer such as styrene or vinylbenzoic acid, and, if necessary, a monomer copolymerizable therewith, such as acrylamide, and a radical polymerization initiator, or a radical polymerization initiator and a chain transfer agent (also referred to as a molecular weight modifier), are charged into a reaction vessel and polymerized under an inert gas atmosphere at 50°C to 120°C for 3 to 20 hours; and a sequential addition method in which the monomer and molecular weight modifier or a solution thereof, and a polymerization initiator are continuously supplied to a reaction vessel while polymerization is carried out.
Among these, the sequential addition method is preferred in terms of excellent removal of polymerization heat and molecular weight controllability, and among the sequential addition methods, the so-called power feed polymerization method, in which the composition of the added monomers is continuously changed, is preferred in terms of excellent uniformity of the copolymer composition.

The radical polymerization initiator used here is the same as that described above, but a hydrophilic azo radical polymerization initiator is particularly preferred. When a peroxide is used as the radical polymerization initiator, a reducing agent such as ascorbic acid, erythorbic acid, aniline, tertiary amine, Rongalite, hydrosulfite, sodium sulfite, sodium hydrogen sulfite, sodium thiosulfate, or sodium hypophosphite may be used in combination.
The amount of the radical polymerization initiator used depends on the type of radical initiator and the polymerization method, but is usually 0.01 mol % to 100.00 mol % based on the total monomers. However, taking into consideration the purity of the resulting polymer, the amount is preferably 0.01 mol % to 20.00 mol %, and more preferably 0.10 mol % to 15.00 mol %.

The chain transfer agent (molecular weight regulator) is not particularly limited, and examples thereof include thioglycolic acid, thiomalic acid, 2-mercaptopropionic acid, 3-mercaptopropionic acid, thiosalicylic acid, 3-mercaptobenzoic acid, 4-mercaptobenzoic acid, thiomalonic acid, dithiosuccinic acid, thiomaleic acid, thiomaleic anhydride, dithiomaleic acid, thioglutaric acid, cysteine, homocysteine, 5-mercaptotetrazoleacetic acid, 3-mercapto-1-propanesulfonic acid, 3-mercaptopropane-1,2-diol, mercaptoethanol, 1-mercapto-2-methylpropanol ... mercaptans such as 2-dimethylmercaptoethane, 2-mercaptoethylamine hydrochloride, 6-mercapto-1-hexanol, 2-mercapto-1-imidazole, 3-mercapto-1,2,4-triazole, cysteine, N-acylcysteine, glutathione, N-butylaminoethanethiol, and N,N-diethylaminoethanethiol, diisopropyl xanthogen disulfide, diethyl xanthogen disulfide, diethylthiuram disulfide, 2,2'-dithiodipropionic acid, 3,3'-dithiodi Examples of the alkyl iodide include disulfides such as propionic acid, 4,4'-dithiodibutanoic acid, and 2,2'-dithiobisbenzoic acid; halogenated hydrocarbons such as iodoform; alkyl iodides such as α-iodobenzyl cyanide, 1-iodoethylbenzene, ethyl 2-iodo-2-phenylacetate, 2-iodo-2-phenylacetic acid, 2-iodopropanoic acid, and 2-iodoacetic acid; diphenylethylene, p-chlorodiphenylethylene, p-cyanodiphenylethylene, α-methylstyrene dimer, organic tellurium compounds, and sulfur.
Among these, thioglycolic acid, thiomalic acid, 2-mercaptopropionic acid, 3-mercaptopropionic acid, thiomalonic acid, dithiosuccinic acid, thiomaleic acid, thiomaleic anhydride, dithiomaleic acid, thioglutaric acid, cysteine, homocysteine, 5-mercaptotetrazoleacetic acid, 3-mercapto-1-propanesulfonic acid, 3-mercaptopropane-1,2-diol, mercaptoethanol, 1 Hydrophilic chain transfer agents such as 2-dimethylmercaptoethane, 2-mercaptoethylamine hydrochloride, 6-mercapto-1-hexanol, 2-mercapto-1-imidazole, 3-mercapto-1,2,4-triazole, cysteine, N-acylcysteine, glutathione, N-butylaminoethanethiol, and N,N-diethylaminoethanethiol are preferred, and hydrophilic chain transfer agents containing a carboxyl group such as thioglycolic acid, thiomalic acid, 2-mercaptopropionic acid, 3-mercaptopropionic acid, thiomalonic acid, dithiosuccinic acid, thiomaleic acid, thiomaleic anhydride, dithiomaleic acid, thioglutaric acid, cysteine, homocysteine, 5-mercaptotetrazoleacetic acid, and 3-mercapto-1-propanesulfonic acid are more preferred from the viewpoint of improving phase separation.

The total proportion of polymerization initiator, RAFT agent, and chain transfer agent relative to all monomers is preferably 0.90 mol% to 13.00 mol% in order to achieve the target UCST behavior and low viscosity, and more preferably 2.00 mol% to 13.00 mol% in order to achieve an even lower viscosity.

The polymerization solvent can be the same as that described above for RAFT polymerization.

In addition, the polymers obtained by the traditional radical polymerization described above can be purified using the methods described above for RAFT polymerization.

Another method for producing carboxylic acid polymers involves radical or anionic copolymerization of a vinyl monomer containing a carboxylic acid ester group, such as an acrylic ester, methacrylic ester, or maleic ester, with an aromatic vinyl monomer, followed by hydrolysis of the carboxylic acid ester with an acid or base to regenerate the carboxyl group. Alternatively, radical or anionic copolymerization of acrylic acid, methacrylic acid, or their esters with an aromatic vinyl monomer, such as para-t-butoxystyrene, para-acetoxystyrene, or para-1-ethoxyethoxystyrene, followed by hydrolysis with an acid such as hydrochloric acid or sulfuric acid can regenerate the carboxyl group and hydroxystyrene structural units. For anionic copolymerization, solvents containing no active hydrogen, such as toluene, xylene, hexane, cyclohexane, tetrahydrofuran, dioxane, or mixtures thereof, are used. Known initiators, such as alkyllithium, sodium naphthalene, and alkyllithium/bis(2,6-di-t-butylphenoxy)methylaluminum, can be used. The polymerization initiator is adjusted according to the target polymer molecular weight. The polymerization temperature is -30°C to 40°C, and -30°C to 10°C is more preferable to suppress side reactions.

When performing radical polymerization, it is preferable to use a polymerization initiator, molecular weight modifier, or chain transfer agent containing a carboxyl group, as described above. The same is true for anionic polymerization; from the standpoint of phase separation, it is preferable to introduce carboxyl groups by methods such as treating the polymer growing end with carbon dioxide gas.

The narrower the molecular weight distribution of the polymer and the fewer low-molecular-weight impurities it contains, the better its temperature response and phase separation properties will be, so it is best to purify it using the method described above.

When the carboxylic acid polymer of the present invention is used as a driving solution for a forward osmosis membrane water treatment system or osmotic power generation system, the viscosity and UCST of the driving solution can be adjusted according to the operating conditions by adding a surfactant, water-soluble polymer, etc.

The present invention will be explained in more detail using the following examples, but the present invention is not limited to these examples in any way.

The analytical instruments and measurement methods used in this example are listed below.
1. Measurement of polymerization conversion rate and molecular weight by gel permeation chromatography (GPC) The reaction solutions before and after polymerization were dissolved in the following eluents, and GPC measurement was carried out under the following conditions.
The polymerization conversion was calculated from a calibration curve prepared using each monomer, and the molecular weight of the polymer was calculated from a calibration curve prepared using standard polystyrene sodium sulfonate or standard polyethylene oxide.
Model: Tosoh Corporation HLC-8320
Column: TSK guard column AW-H/TSK AW-6000/TSK AW-3000/TSK AW-2500
Eluent: 0.2 M aqueous sodium sulfate solution/acetonitrile = 65/35 (volume ratio) Solution flow rate: 0.6 ml/min, injection volume: 10 μl, column temperature: 40°C
Detector: UV detector (wavelength 210 nm) or RI detector. Polymer calibration curve: Created from peak top molecular weight and elution time using standard polystyrene sodium sulfonate (manufactured by Sowa Scientific). For polyampholite, standard polyethylene oxide (manufactured by Sigma-Aldrich) was used.

2. Structural Analysis of Polymer by ¹³ C-NMR ¹³ C-NMR analysis of the polymer was carried out under the following conditions to confirm the copolymer composition and terminal structure.
Model: Bruker AVANCE NEO 700
Probe: 10mmφ PABBO BB
Observed nucleus: ^13C (176.07MHz)
Solvent: dimethyl sulfoxide-d6
Concentration: about 10%
Number of times accumulated: 2048 Temperature: room temperature Reference: tetramethylsilane (TMS)
Measurement mode: Inverse gated decoupling (quantitative measurement method)

3. Measurement of UCST of aqueous polymer solution Polymer and ion exchange were placed in a glass bottle to prepare an aqueous polymer solution of a predetermined concentration. The contents were heated in an oil bath while stirring with a magnetic stirrer. When the solution became hydrophilic and transparent due to heating and dissolution, it was cooled to become hydrophobic and opaque, and then reheated. The temperature at which the solution became hydrophilic again and transparent was taken as the UCST.

4. Measurement of osmotic pressure of aqueous polymer solution The water activity value of the aqueous polymer solution was converted to osmotic pressure (bar) using the following conversion formula [see Divina D.; Separation and Purification Technology 138 (2014) 92-97].
Water activity was measured at 50°C using a water activity measuring device (AquaLab Series 4TDL manufactured by Inex Co., Ltd.). Measurements were performed three times, and the average value was used to calculate the osmotic pressure. To minimize measurement errors, the water activity measuring device was placed in a thermostatic bath at 50°C.

5. Viscosity of Aqueous Polymer Solution: The viscosity was measured at a predetermined temperature using a Brookfield viscometer LVDV2T (manufactured by Eiko Seiki Co., Ltd.).

<Reagents used>
The compounds described in the examples below were used, but the present invention is not limited to these examples in any way.
AA: acrylic acid (purity 99%, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.)
MAA: methacrylic acid (purity 99%, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.)
St: styrene (purity 99%, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.)
αMSt: α-methylstyrene (purity 99%, manufactured by Tokyo Chemical Industry Co., Ltd.)
4-VBA: 4-vinylbenzoic acid (purity 97%, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.)
AM: Acrylamide (purity 99%, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.)
MAM: methacrylamide (purity 97%, Fujifilm Wako Pure Chemical Industries, Ltd.)
NaSS: sodium p-styrenesulfonate (purity 98%, manufactured by Tokyo Chemical Industry Co., Ltd.)
VBTAC: vinylbenzyltrimethylammonium chloride (purity 99%, manufactured by Sigma-Aldrich)
V-501: 4,4'-azobis-(4-cyanopentanoic acid) (purity 98%, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.)
AIBN: 2,2'-azobis(isobutyronitrile) (purity 98%, 2,2'-azobis(isobutyronitrile))
HP: Hydrogen peroxide (30% purity, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.)
TGL: 3-mercapto-1,2-propanediol (purity 97%, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.)
TMA: Thiomalic acid (purity 98%, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.)
RAFT agent—1: 3-((((1-carboxyethyl)thio)carbonothioyl)thio)propanoic acid (purity 95%, Sigma-Aldrich)

The structural units (A), (B), and (C) shown below refer to the structural units in the above formulas (1), (2), and (3), respectively.

Example 1
<Synthesis of AA/St Copolymer>
A 1000 mL glass four-neck flask equipped with a nitrogen inlet tube, a three-way stopcock, and a Dimroth condenser was charged with acrylic acid (AA) (41.60 g, 571.52 mmol), styrene (St) (9.10 g, 86.46 mmol), RAFT agent-1 (3.50 g, 13.07 mmol), initiator V-501 (0.30 g, 1.05 mmol), isopropanol (150.00 g), and ion-exchanged water (150.00 g) and dissolved therein to obtain a homogeneous solution (the amount of St charged relative to the total amount of AA and St=13.14 mol%).
This solution was thoroughly degassed by repeatedly reducing the pressure with an aspirator and introducing nitrogen, and then polymerized at 70° C. for 20 hours under a nitrogen atmosphere while stirring with a magnetic stirrer.
The polymerization conversion rates at the end of the reaction measured by GPC were AA = 82%, St = 100%, number average molecular weight Mn = 3,300, weight average molecular weight Mw = 4,700, and Mw/Mn = 1.42. That is, from the polymerization conversion rates of each monomer, the content of St (structural unit (B)) relative to the total of AA (structural unit (A)) and St (structural unit (B)) contained in the polymer was 15 mol%. The polymerization solution was vacuum dried at 85°C for 5 hours to remove the solvent and unreacted monomers, yielding 44.26 g of a powdery polymer.

<Physical properties of copolymer>
The relationship between the concentration of the polymer in aqueous solution and the UCST is shown in Table 1. It is clear that the UCST is lower and the slope of the phase separation curve is smaller than those of the polyampholite of Comparative Example 1, which utilizes strong electrostatic interactions; Comparative Example 3, which has a high content of structural unit (B) in the polymer; Comparative Example 4, which has a wide molecular weight distribution; Comparative Example 5, which has a high molecular weight; and Comparative Examples 6 and 7, which used a different polymerization initiator. On the other hand, Comparative Example 2, which had too little styrene, a hydrophobic component, dissolved in water over a wide concentration range and did not exhibit the target UCST.
The viscosity of a 50 wt % aqueous solution of the polymer was measured at various temperatures (No. 3 rotor, 60 rpm), and the results are shown in Table 2 below. It is clear that the viscosity of Comparative Example 1, which utilizes strong electrostatic interaction, is significantly lower than that of the polyampholite of Comparative Example 1.
This polymer is a weak electrolyte and is expected to have a lower osmotic pressure than the conventional strong electrolyte polyampholite (Comparative Example 1). However, because it has an extremely low viscosity (concentration polarization is expected to be less likely to occur), it is expected to be used as a driving solution for forward osmosis membrane water treatment systems and osmotic pressure power generation.

Example 2
<Synthesis of AA/VBA copolymer>
Acrylic acid (AA) (41.55 g, 570.84 mmol), 4-vinylbenzoic acid (VBA) (15.60 g, 105.08 mmol), RAFT agent-1 (3.90 g, 14.57 mmol), initiator V-501 (0.60 g, 2.10 mmol), isopropanol (225.00 g), and ion-exchanged water (225.00 g) were charged into a 1000 mL glass four-neck flask equipped with a nitrogen inlet tube, a three-way stopcock, and a Dimroth condenser, and dissolved therein to prepare a homogeneous solution (the amount of VBA charged relative to the total amount of AA and VBA = 15.55 mol%).
This solution was thoroughly degassed by repeatedly reducing the pressure with an aspirator and introducing nitrogen, and then polymerized at 70° C. for 20 hours under a nitrogen atmosphere while stirring with a magnetic stirrer.
The polymerization conversion rates at the end of the reaction, as measured by GPC, were AA = 87%, VBA = 100%, number average molecular weight Mn = 3,800, weight average molecular weight Mw = 5,200, and Mw/Mn = 1.37. That is, from the polymerization conversion rates of each monomer, the content of VBA (structural unit (B)) relative to the total of AA (structural unit (A)) and VBA (structural unit (B)) contained in the polymer was 17 mol%. The polymerization solution was vacuum dried at 85°C for 5 hours to remove the solvent and unreacted monomers, yielding 52.96 g of a powdery polymer.

<Physical properties of copolymer>
The relationship between the concentration of the polymer in aqueous solution and the UCST is shown in Table 2. It is clear that the UCST is lower, the slope of the phase separation curve is smaller, and the viscosity of a 50 wt % aqueous polymer solution is significantly lower than in Comparative Examples 1 and 2 to 8 described below.
This polymer is a weak electrolyte and is expected to have a lower osmotic pressure than the conventional strong electrolyte polyampholite (Comparative Example 1). However, because it has an extremely low viscosity (concentration polarization is expected to be less likely to occur), it is expected to be used as a driving solution for forward osmosis membrane water treatment systems and osmotic pressure power generation.

Example 3
<Synthesis of AA/VBA copolymer>
Acrylic acid (AA) (42.00 g, 577.02 mmol), 4-vinylbenzoic acid (VBA) (9.00 g, 60.62 mmol), RAFT agent-1 (1.00 g, 3.74 mmol), initiator V-501 (0.60 g, 2.10 mmol), isopropanol (225.00 g), and ion-exchanged water (225.00 g) were charged into a 1000 mL glass four-neck flask equipped with a nitrogen inlet tube, a three-way stopcock, and a Dimroth condenser, and dissolved therein to prepare a homogeneous solution (the amount of VBA charged relative to the total amount of AA and VBA = 9.51 mol%).
This solution was thoroughly degassed by repeatedly reducing the pressure with an aspirator and introducing nitrogen, and then polymerized at 70° C. for 20 hours under a nitrogen atmosphere while stirring with a magnetic stirrer.
The polymerization conversion rates at the end of the reaction, as measured by GPC, were AA = 91%, VBA = 100%, number average molecular weight Mn = 12,900, weight average molecular weight Mw = 20,500, and Mw/Mn = 1.59. That is, from the polymerization conversion rates of each monomer, the content of VBA (structural unit (B)) relative to the total of AA (structural unit (A)) and VBA (structural unit (B)) contained in the polymer was 10 mol%. The polymerization solution was vacuum dried at 85°C for 5 hours to remove the solvent and unreacted monomers, yielding 46.81 g of a powdery polymer.

<Physical properties of copolymer>
The relationship between the concentration of the polymer in aqueous solution and the UCST is shown in Table 2. It is clear that compared with Comparative Examples 1 and 2 to 8, the UCST was lower, the slope of the phase separation curve was smaller, and furthermore the viscosity of a 50 wt % polymer aqueous solution was significantly lower.
This polymer is a weak electrolyte and is expected to have a lower osmotic pressure than the conventional strong electrolyte polyampholite (Comparative Example 1). However, because it has an extremely low viscosity (concentration polarization is expected to be less likely to occur), it is expected to be used as a driving solution for forward osmosis membrane water treatment systems and osmotic pressure power generation.

Example 4
<Synthesis of AA/VBA copolymer>
Acrylic acid (AA) (30.00 g, 412.16 mmol), 4-vinylbenzoic acid (VBA) (15.00 g, 101.04 mmol), RAFT agent-1 (5.00 g, 18.68 mmol), initiator V-501 (1.00 g, 2.10 mmol), isopropanol (150.00 g), and ion-exchanged water (150.00 g) were charged into a 1000 mL glass four-neck flask equipped with a nitrogen inlet tube, a three-way stopcock, and a Dimroth condenser, and dissolved therein to prepare a homogeneous solution (the amount of VBA charged relative to the total amount of AA and VBA=19.69 mol%).
This solution was thoroughly degassed by repeatedly reducing the pressure with an aspirator and introducing nitrogen, and then polymerized at 70° C. for 20 hours under a nitrogen atmosphere while stirring with a magnetic stirrer.
The polymerization conversion rates at the end of the reaction, as measured by GPC, were AA = 72%, VBA = 100%, number average molecular weight Mn = 2500, weight average molecular weight Mw = 4300, and Mw/Mn = 1.72. That is, from the polymerization conversion rates of each monomer, the content of VBA (structural unit (B)) relative to the total of AA (structural unit (A)) and VBA (structural unit (B)) contained in the polymer was 25 mol%. The polymerization solution was vacuum dried at 85°C for 5 hours to remove the solvent and unreacted monomers, yielding 39.25 g of a powdery polymer.

<Physical properties of copolymer>
The relationship between the concentration of the polymer in aqueous solution and the UCST is shown in Table 2. It is clear that compared with Comparative Examples 1 and 2 to 8, the UCST was lower, the slope of the phase separation curve was smaller, and furthermore the viscosity of a 50 wt % polymer aqueous solution was significantly lower.
This polymer is a weak electrolyte and is expected to have a lower osmotic pressure than the conventional strong electrolyte polyampholite (Comparative Example 1). However, because it has an extremely low viscosity (concentration polarization is expected to be less likely to occur), it is expected to be used as a driving solution for forward osmosis membrane water treatment systems and osmotic pressure power generation.

Example 5
<Synthesis of AA/AM/VBA Copolymer>
A 1000 mL four-neck glass flask equipped with a nitrogen inlet tube, a three-way stopcock, and a Dimroth condenser was charged with acrylic acid (AA) (40.00 g, 549.54 mmol), 4-vinylbenzoic acid (VBA) (7.00 g, 66.51 mmol), acrylamide (AM) (2.00 g, 27.86 mmol), RAFT agent-1 (3.30 g, 12.33 mmol), initiator V-501 (0.30 g, 1.05 mmol), isopropanol (150.00 g), and ion-exchanged water (150.00 g) and dissolved therein to prepare a homogeneous solution (the amount of VBA charged relative to the total of AA and VBA=10.80 mol %, the amount of AM charged relative to the total of all monomers=4.33 mol %).
This solution was thoroughly degassed by repeatedly reducing the pressure with an aspirator and introducing nitrogen, and then polymerized at 70° C. for 20 hours under a nitrogen atmosphere while stirring with a magnetic stirrer.
The polymerization conversion rates at the end of the reaction, as measured by GPC, were AA = 81%, AM = 83%, VBA = 100%, number average molecular weight Mn = 3,800, weight average molecular weight Mw = 5,400, and Mw/Mn = 1.42. That is, from the polymerization conversion rates of each monomer, the content of VBA (structural unit (B)) relative to the total of AA (structural unit (A)) and VBA (structural unit (B)) contained in the polymer was 13 mol%, and the content of AM (structural unit (C)) relative to the total of structural units (A) to C) was 4 mol%. The polymerization solution was vacuum-dried at 85°C for 5 hours to remove the solvent and unreacted monomers, yielding 44.42 g of a powdery polymer.

<Physical properties of copolymer>
The relationship between the concentration of the polymer in aqueous solution and the UCST is shown in Table 2. It is clear that compared with Comparative Examples 1 and 2 to 8, the UCST was lower, the slope of the phase separation curve was smaller, and furthermore the viscosity of a 50 wt % polymer aqueous solution was significantly lower.
The viscosity of a 50% by weight aqueous solution of the polymer was measured at various temperatures (No. 3 rotor, 60 rpm), and the results are shown in Table 1. It is clear that the viscosity is significantly lower than that of the polyampholite of Comparative Example 1.
This polymer is a weak electrolyte and is expected to have a lower osmotic pressure than the conventional strong electrolyte polyampholite (Comparative Example 1). However, because it has an extremely low viscosity (concentration polarization is expected to be less likely to occur), it is expected to be used as a driving solution for forward osmosis membrane water treatment systems and osmotic pressure power generation.

Example 6
<Synthesis of AA/St Copolymer>
A monomer solution consisting of acrylic acid (AA) (43.71 g, 600.51 mmol), styrene (St) (7.06 g, 67.08 mmol), and 1-propanol (217.00 g), and an initiator solution consisting of initiator V-501 (17.97 g, 62.85 mmol), 1-propanol (285.35 g), and ion-exchanged water (111.82 g) were thoroughly degassed by repeatedly reducing the pressure with an aspirator and introducing nitrogen. A 1000 mL glass four-neck flask equipped with a nitrogen inlet tube, a three-way stopcock, a Dimroth condenser, and a magnetic stirrer was immersed in a 96 °C oil bath, and the monomer solution and initiator solution were added dropwise over 4.5 hours, followed by further heating for 1 hour to polymerize (the amount of St charged relative to the total of AA and St = 10.05 mol%).
The polymerization conversion rates at the end of the reaction, as measured by GPC, were AA = 96%, St = 100%, number average molecular weight Mn = 1,800, weight average molecular weight Mw = 3,100, and Mw/Mn = 1.72. That is, from the polymerization conversion rates of each monomer, the content of St (structural unit (B)) relative to the total of AA (structural unit (A)) and St (structural unit (B)) contained in the polymer was 10 mol%. The polymerization solution was vacuum dried at 85°C for 5 hours to remove the solvent and unreacted monomers, yielding 45.04 g of a powdery polymer.

<Purification of copolymer>
The entire amount of the powdered polymer was added to a glass bottle with ion-exchanged water to prepare a 20 wt% aqueous solution, which was then heated and dissolved at 85°C, cooled to 25°C, and allowed to stand overnight. The supernatant was discarded, and the concentrated solution in the lower layer was recovered. Approximately 1 g of the concentrated solution was accurately weighed into a weighing bottle and vacuum-dried at 85°C for 7 hours. As a result, the solid content was 53 wt% and the polymer recovery rate was approximately 80 wt%.

< ^13C -NMR structural analysis of copolymer>
The ¹³ C-NMR spectrum of the above-mentioned once-purified polymer was measured (FIGS. 3, 4 and 5), and each peak was assigned as shown in Table 1 below based on simulation (simulation software: ACD/C+H NMR Predictors).

The integral per carbon of the acrylic acid structural unit is
G-(B+D)/2-(A+C)/2=92.74
The integral value per carbon atom of the styrene structural unit was (E+F)/6=12.02, and therefore the molar ratio of the acrylic acid structural unit to the styrene structural unit was 89:11 mol%, which was almost consistent with the composition ratio calculated from the GPC conversion rate.
Furthermore, of the two peaks derived from the methyl group and cyano group originating from the polymerization initiator V-501, the broad peak was presumed to be derived from the initiator segment (molecular weight 126.13) attached to the polymer end, as expressed by the following formula (5). Therefore, the molar ratio of acrylic acid structural units, styrene structural units, and initiator segments attached to the polymer end was 92.74:12.02:5.53=16.8:2.2:1. It is believed that the desired UCST behavior was manifested not only by the polymer composition and molecular weight, but also by the effect of the carboxyl group attached to the polymer end.

The peaks derived from the ester structure in Table 3 should not be present in the raw material, and the reason for their appearance is unknown. Furthermore, the initiator residues are derived from decomposition products that did not bond to the polymer ends.

<Physical properties of copolymer>
The relationship between the concentration of the aqueous solution of the polymer purified once and the UCST is shown in Table 1. It is clear that compared with Comparative Examples 1 and 2 to 8, the UCST is lower, the slope of the phase separation curve is smaller, and the viscosity of a 50 wt % aqueous polymer solution is significantly lower.
This polymer is a weak electrolyte and is expected to have a lower osmotic pressure than the conventional strong electrolyte polyampholite (Comparative Example 1). However, because it has an extremely low viscosity (concentration polarization is expected to be less likely to occur), it is expected to be used as a driving solution for forward osmosis membrane water treatment systems and osmotic pressure power generation.

Example 7
<Synthesis of AA/St Copolymer>
A monomer solution consisting of acrylic acid (AA) (40.00 g, 549.54 mmol), styrene (St) (8.70 g, 82.66 mmol), and 1-propanol (230.00 g), and an initiator solution consisting of initiator V-50 (5.00 g, 18.07 mmol), thiomalic acid (4.50 g, 29.37 mmol), and ion-exchanged water (100.00 g), were thoroughly degassed by repeatedly reducing the pressure with an aspirator and introducing nitrogen. A 1000 mL glass four-neck flask equipped with a nitrogen inlet tube, a three-way stopcock, a Dimroth condenser, and a magnetic stirrer was immersed in an oil bath at 96 ° C., and the monomer solution and initiator solution were added dropwise over 5 hours, followed by further heating for 1 hour to polymerize (the amount of St charged relative to the total of AA and St = 13.07 mol%).
The polymerization conversion rates at the end of the reaction, as measured by GPC, were AA = 97%, St = 100%, number average molecular weight Mn = 2700, weight average molecular weight Mw = 4800, and Mw/Mn = 1.78. That is, from the polymerization conversion rates of each monomer, the content of St (structural unit (B)) relative to the total of AA (structural unit (A)) and St (structural unit (B)) contained in the polymer was 13 mol%. The polymerization solution was vacuum dried at 85°C for 5 hours to remove the solvent and unreacted monomers, yielding 45.01 g of a powdery polymer.

<Physical properties of copolymer>
The relationship between the concentration of the polymer in aqueous solution and the UCST is shown in Table 2. It is clear that compared with Comparative Examples 1 and 2 to 8, the UCST was lower, the slope of the phase separation curve was smaller, and furthermore the viscosity of a 50 wt % polymer aqueous solution was significantly lower.
The viscosity of a 50 wt % aqueous solution of the polymer was measured at various temperatures (No. 3 rotor, 60 rpm), and the results are shown in Table 1. It is clear that the viscosity is higher than in Examples 1, 2, and 5, but is significantly lower than that of the polyampholite of Comparative Example 1.
This polymer is a weak electrolyte and is expected to have a lower osmotic pressure than the conventional strong electrolyte polyampholite (Comparative Example 1). However, because it has an extremely low viscosity (concentration polarization is expected to be less likely to occur), it is expected to be used as a driving solution for forward osmosis membrane water treatment systems and osmotic pressure power generation.

Example 8
<Synthesis of MAA/St Copolymer-1>
A monomer solution consisting of methacrylic acid (MAA) (52.20 g, 600.49 mmol), styrene (St) (7.00 g, 66.51 mmol), and 1-propanol (200.00 g), and an initiator solution consisting of initiator V-50 (23.00 g, 83.12 mmol), 1-propanol (350.00 g), and ion-exchanged water (135.00 g) were thoroughly degassed by repeatedly reducing the pressure with an aspirator and introducing nitrogen. A 1000 mL glass four-neck flask equipped with a nitrogen inlet tube, a three-way stopcock, a Dimroth condenser, and a magnetic stirrer was immersed in an oil bath at 96 °C, and the monomer solution and initiator solution were added dropwise over 6 hours, followed by further heating for 1 hour to polymerize (the amount of St charged relative to the total of MAA and St = 9.97 mol%).
The polymerization conversion rates at the end of the reaction, as measured by GPC, were MAA = 92%, St = 100%, number average molecular weight Mn = 1900, weight average molecular weight Mw = 2400, and Mw/Mn = 1.26. That is, from the polymerization conversion rates of each monomer, the content of St (structural unit (B)) relative to the total of MAA (structural unit (A)) and St (structural unit (B)) contained in the polymer was 11 mol%. The polymerization solution was vacuum dried at 85°C for 5 hours to remove the solvent and unreacted monomers, yielding 55.01 g of a powdery polymer.

<Physical properties of copolymer>
The relationship between the concentration of the polymer in an aqueous solution and the UCST is shown in Table 2. Compared with the acrylic acid-based solutions of Examples 1 to 7, the UCST and viscosity of the aqueous solution were higher, but compared with Comparative Examples 1 and 2 to 8, the UCST was lower, the slope of the phase separation curve was smaller, and furthermore, it is clear that the viscosity of a 50 wt% aqueous polymer solution was significantly lower.
This polymer is a weak electrolyte and is expected to have a lower osmotic pressure than the conventional strong electrolyte polyampholite (Comparative Example 1). However, because it has an extremely low viscosity (concentration polarization is expected to be less likely to occur), it is expected to be used as a driving solution for forward osmosis membrane water treatment systems and osmotic pressure power generation.

Comparative Example 1
<Synthesis of NaSS/VBTAC Copolymer>
A 1000 mL four-neck glass flask equipped with a nitrogen inlet tube, a three-way stopcock, and a Dimroth condenser was charged with 55.00 g of ion-exchanged water. 4-Vinylbenzyltrimethylammonium chloride (VBTAC) (54.98 g, 254.48 mmol), sodium styrenesulfonate (NaSS) (59.17 g, 254.82 mmol), thioglycerol (TGL) (1.54 g, 13.95 mmol), and water-soluble azo initiator V-50 (6.80 g, 24.57 mmol) were dissolved in 551.30 g of ion-exchanged water. The monomer solution was degassed by repeatedly aspirating and introducing nitrogen. The solution was added dropwise over 3 hours while being heated and polymerized at 85°C, followed by further aging at 85°C for 2 hours.
The polymerization conversion at the end of the reaction measured by GPC was VBTAC = 100%, NaSS = 100%, number average molecular weight Mn = 860, weight average molecular weight Mw = 2200, Mw/Mn = 2.56. The polymerization solution was concentrated using a rotary evaporator, and the physical properties of the polymer were confirmed.

<Physical properties of copolymer>
The viscosity of a 50 wt % aqueous solution of this polymer was measured at various temperatures, and the results are shown in Table 1 (No. 4 rotor, 30 rpm). It is clear that, despite its low molecular weight and the inclusion of NaCl as a counter ion, it had a significantly higher viscosity than the Examples. This polymer contains NaCl in amounts equivalent to the anions and cations in the polymer. However, if the NaCl is removed by dissolution/phase separation, the charge shielding effect of NaCl disappears, and the electrostatic interaction between the polymers increases, presumably resulting in a further increase in the solution viscosity.
Although the polymer does indeed exhibit a UCST, because it contains NaCl, using it as is as a driving agent in a forward osmosis membrane water treatment system will result in water containing polymer-derived NaCl, rather than fresh water, at least initially. Therefore, an equal volume of ion-exchanged water was added to a 50 wt% aqueous solution of the polymer, and the mixture was stirred and heated at 85°C. The mixture was then allowed to stand to allow phase separation, and the supernatant containing NaCl was discarded. An equal volume of ion-exchanged water to the remaining concentrated layer was added, and the mixture was again stirred and heated at 85°C. The mixture was then allowed to stand to allow phase separation, and the supernatant was discarded. This purification procedure was repeated a total of four times, after which the UCST of the polymer was confirmed and the results are shown in Table 3. It is clear that the UCST was higher and the slope of the phase separation curve was steeper than in the Examples.

Comparative Example 2
<Synthesis of AA/St Copolymer>
Acrylic acid (AA) (48.00 g, 659.45 mmol), styrene (St) (3.00 g, 28.50 mmol), RAFT agent-1 (3.20 g, 11.95 mmol), initiator AIBN (0.20 g, 1.19 mmol), isopropanol (150.00 g), and ion-exchanged water (150.00 g) were charged into a 1000 mL glass four-neck flask equipped with a nitrogen inlet tube, a three-way stopcock, and a Dimroth condenser, and dissolved therein to prepare a homogeneous solution (the amount of St charged relative to the total amount of AA and St=4.14 mol%).
This solution was thoroughly degassed by repeatedly reducing the pressure with an aspirator and introducing nitrogen, and then polymerized at 70° C. for 20 hours under a nitrogen atmosphere while stirring with a magnetic stirrer.
The polymerization conversion rates at the end of the reaction measured by GPC were AA = 93%, St = 100%, number average molecular weight Mn = 4200, weight average molecular weight Mw = 6200, and Mw/Mn = 1.48. That is, from the polymerization conversion rates of each monomer, the content of St (structural unit (B)) relative to the total of AA (structural unit (A)) and St (structural unit (B)) contained in the polymer was 4 mol%. The polymerization solution was vacuum dried at 85°C for 5 hours to remove the solvent and unreacted monomers, yielding 46.46 g of a powdery polymer.

<Physical properties of copolymer>
As shown in Table 3, all of the 30 wt % to 50 wt % aqueous solutions of the polymer became homogeneous solutions at room temperature, and UCST was not observed above room temperature. This is because the structural unit (B) in the polymer was less than in the Examples (Table 1), and the hydrophobic interaction was too weak.
Comparative Example 3
<Synthesis of AA/VBA copolymer>
A 1000 mL glass four-neck flask equipped with a nitrogen inlet tube, a three-way stopcock, and a Dimroth condenser was charged with acrylic acid (AA) (12.00 g, 164.86 mmol), 4-vinylbenzoic acid (VBA) (15.00 g, 101.04 mmol), RAFT agent-1 (1.50 g, 5.60 mmol), initiator V-501 (0.15 g, 0.52 mmol), isopropanol (150.00 g), and ion-exchanged water (150.00 g) and dissolved therein to prepare a homogeneous solution (the amount of VBA charged relative to the total amount of AA and VBA = 38.00 mol%).
This solution was thoroughly degassed by repeatedly reducing the pressure with an aspirator and introducing nitrogen, and then polymerized at 70° C. for 20 hours under a nitrogen atmosphere while stirring with a magnetic stirrer.
The polymerization conversion rates at the end of the reaction, as measured by GPC, were AA = 71%, VBA = 100%, number average molecular weight Mn = 4,800, weight average molecular weight Mw = 7,500, and Mw/Mn = 1.56. That is, from the polymerization conversion rates of each monomer, the content of VBA (structural unit (B)) relative to the total of AA (structural unit (A)) and VBA (structural unit (B)) contained in the polymer was 46 mol%. The polymerization solution was vacuum dried at 85°C for 5 hours to remove the solvent and unreacted monomers, yielding 24.98 g of a powdery polymer.

<Physical properties of copolymer>
As shown in Table 3, the polymer exhibited temperature responsiveness, but the UCST was clearly too high compared to the examples. This was because the polymer contained a large amount of structural unit (B), resulting in too strong hydrophobic interactions.

Comparative Example 4
<Synthesis of AA/VBA copolymer>
Acrylic acid (AA) (41.55 g, 570.84 mmol), 4-vinylbenzoic acid (VBA) (15.60 g, 105.08 mmol), RAFT agent-1 (0.70 g, 2.61 mmol), initiator V-501 (0.60 g, 0.2.10 mmol), isopropanol (225.00 g), and ion-exchanged water (225.00 g) were charged into a 1000 mL glass four-neck flask equipped with a nitrogen inlet tube, a three-way stopcock, and a Dimroth condenser, and dissolved therein to prepare a homogeneous solution (the amount of VBA charged relative to the total amount of AA and VBA = 15.55 mol%).
This solution was thoroughly degassed by repeatedly reducing the pressure with an aspirator and introducing nitrogen, and then polymerized at 70° C. for 20 hours under a nitrogen atmosphere while stirring with a magnetic stirrer.
The polymerization conversion rates at the end of the reaction, as measured by GPC, were AA = 89%, VBA = 100%, number average molecular weight Mn = 20,800, weight average molecular weight Mw = 33,100, and Mw/Mn = 1.59. That is, from the polymerization conversion rates of each monomer, the content of VBA (structural unit (B)) relative to the total of AA (structural unit (A)) and VBA (structural unit (B)) contained in the polymer was 17 mol%. The polymerization solution was vacuum dried at 85°C for 5 hours to remove the solvent and unreacted monomers, yielding 52.98 g of a powdery polymer.

<Physical properties of copolymer>
As shown in Table 3, the polymer exhibited temperature responsiveness, but the UCST was clearly too high compared to the examples. This was because the polymer contained a large amount of structural unit (B), resulting in too strong hydrophobic interactions.

Comparative Example 5
<Synthesis of AA/St Copolymer>
An aqueous solution of acrylic acid (AA) (8.02 g, 110.18 mmol), styrene (St) (1.74 g, 16.53 mmol), 30% hydrogen peroxide solution (15.04 g, 132.67 mmol), 1-propanol (30.36 g), and ion-exchanged water (29.71 g), which had been degassed in advance by the method described above, was slowly added dropwise over 10 hours to a 200 mL four-neck glass flask equipped with a nitrogen inlet tube, a three-way stopcock, and a Dimroth condenser, while polymerization was carried out at 97°C for 20 hours (the amount of St charged relative to the total amount of AA and St=13.05 mol%).
The polymerization conversion rates at the end of the reaction, as measured by GPC, were AA = 70%, St = 100%, number average molecular weight Mn = 7000, weight average molecular weight Mw = 16300, and Mw/Mn = 2.33. That is, from the polymerization conversion rates of each monomer, the content of St (structural unit (B)) relative to the total of AA (structural unit (A)) and St (structural unit (B)) contained in the polymer was 18 mol%. The polymerization solution was vacuum dried at 85°C for 5 hours to remove the solvent and unreacted monomers, yielding 7.26 g of a powdery polymer.

<Physical properties of copolymer>
As shown in Table 3, a 40% to 50% by weight aqueous solution of the polymer did not dissolve and did not exhibit UCST even when heated to 80°C. It is believed that the UCST was increased by the small amount of high molecular weight components contained due to the broad molecular weight distribution.

Comparative Example 6
<Synthesis of AA/St Copolymer>
A monomer solution consisting of acrylic acid (AA) (41.50 g, 570.15 mmol), styrene (St) (6.90 g, 65.56 mmol), and 1-propanol (250.00 g), and an initiator solution consisting of the water-soluble initiator APS (16.50 g, 71.58 mmol) and ion-exchanged water (220.00 g), were thoroughly degassed by repeatedly reducing the pressure with an aspirator and introducing nitrogen. A 1000 mL glass four-neck flask equipped with a nitrogen inlet tube, a three-way stopcock, a Dimroth condenser, and a magnetic stirrer was immersed in a 96 °C oil bath, and the monomer solution and initiator solution were added dropwise over 5 hours, followed by further heating for 1 hour to polymerize (the amount of St charged relative to the total of AA and St = 10.31 mol%).
The polymerization conversion rates at the end of the reaction measured by GPC were AA = 100%, St = 100%, number average molecular weight Mn = 1000, weight average molecular weight Mw = 1900, and Mw/Mn = 1.90. That is, from the polymerization conversion rates of each monomer, the content of St (structural unit (B)) relative to the total of AA (structural unit (A)) and St (structural unit (B)) contained in the polymer was 10 mol%. The polymerization solution was vacuum dried at 85°C for 5 hours to remove the solvent and unreacted monomers, and 49.52 g of a powdery polymer was obtained.

<Physical properties of copolymer>
As shown in Table 3, the polymer had a small molecular weight and was poorly soluble in water, despite the use of a water-soluble radical polymerization initiator. The reason for this is unclear, but it is suggested that this is due to differences in the polymer terminal structure, i.e., the polymer terminal has a hydrophobic group derived from the initiator introduced and does not contain a carboxyl group.

Comparative Example 7
<Synthesis of AA/St Copolymer>
A monomer solution consisting of acrylic acid (AA) (42.10 g, 578.39 mmol), styrene (St) (6.80 g, 64.61 mmol), and 1-propanol (220.00 g), and an initiator solution consisting of oil-soluble initiator V-65 (20.00 g, 78.91 mmol) and 1-propanol (220.00 g) were thoroughly degassed by repeatedly reducing the pressure with an aspirator and introducing nitrogen. A 1000 mL glass four-neck flask equipped with a nitrogen inlet tube, a three-way stopcock, a Dimroth condenser, and a magnetic stirrer was immersed in a 96 °C oil bath, and the monomer solution and initiator solution were added dropwise over 5 hours, followed by further heating for 1 hour to polymerize (the amount of St charged relative to the total of AA and St = 10.05 mol%).
The polymerization conversion rates at the end of the reaction measured by GPC were AA = 94%, St = 100%, number average molecular weight Mn = 700, weight average molecular weight Mw = 1000, and Mw/Mn = 1.43. That is, from the polymerization conversion rates of each monomer, the content of St (structural unit (B)) relative to the total of AA (structural unit (A)) and St (structural unit (B)) contained in the polymer was 11 mol%. The polymerization solution was vacuum dried at 85°C for 5 hours to remove the solvent and unreacted monomers, and 45.38 g of a powdery polymer was obtained.

<Physical properties of copolymer>
As shown in Table 3, the polymer had unclear temperature response and was poorly soluble in water, despite its small molecular weight. The reason for this is not entirely clear, but it is suggested that this is because a hydrophobic group derived from the initiator was introduced to the polymer terminal, i.e., the polymer terminal did not contain a carboxyl group.
Comparative Example 8
<Synthesis of AA/St Copolymer>
A monomer solution consisting of acrylic acid (AA) (50.10 g, 688.30 mmol), styrene (St) (11.00 g, 104.51 mmol), and 1-propanol (220.00 g), and an initiator solution consisting of oil-soluble initiator V-65 (10.00 g, 39.46 mmol), thioglycerol (4.30 g, 38.96 mmol), and 1-propanol (250.00 g), were thoroughly degassed by repeatedly aspirating and introducing nitrogen. A 1000 mL glass four-neck flask equipped with a nitrogen inlet tube, a three-way stopcock, a Dimroth condenser, and a magnetic stirrer was immersed in a 96 °C oil bath, and the monomer solution and initiator solution were added dropwise over 5 hours, followed by further heating for 1 hour to polymerize (the amount of St charged relative to the total of AA and St = 13.18 mol%).
The polymerization conversion rates at the end of the reaction, as measured by GPC, were AA = 93%, St = 96%, number average molecular weight Mn = 1,400, weight average molecular weight Mw = 1,500, and Mw/Mn = 1.07. That is, from the polymerization conversion rates of each monomer, the content of St (structural unit (B)) relative to the total of AA (structural unit (A)) and St (structural unit (B)) contained in the polymer was 14 mol%. The polymerization solution was vacuum dried at 85°C for 5 hours to remove the solvent and unreacted monomers, yielding 58.05 g of a powdery polymer.

<Physical properties of copolymer>
As shown in Table 3, the polymer had unclear temperature response and was poorly soluble in water, despite its small molecular weight. The reason for this is not entirely clear, but it is suggested that the polymer terminals lack hydrophilicity, i.e., the polymer terminals do not contain carboxyl groups.

The carboxylic acid-based upper critical solution temperature (UCST) polymer of the present invention is an ionic polymer with excellent hydrolysis resistance and low viscosity, and is therefore expected to be used as a driving solution in forward osmosis membrane water treatment systems and osmotic power generation systems.

Claims (18)

It consists of the following structural units (A) and (B):
The content of the structural unit (B) is 5 mol % to 30 mol % based on the total of the structural units (A) and (B);
a polymer having a number average molecular weight Mn of 500 to 20,000 daltons (Da) and a ratio Mw/Mn of number average molecular weight Mn to weight average molecular weight Mw of 1.00 to 2.00;
the polymer is water soluble and has an upper critical solution temperature (UCST) ;
a viscosity of a 50 wt % aqueous solution containing the polymer is 10 mPa s to 10,000 mPa s at 25°C;
It is hydrophobic at temperatures below UCST and hydrophilic at temperatures above UCST.
Temperature-responsive carboxylic acid-based polymers.
The structural unit (A) is represented by the following general formula (1):
(In formula (1), _R1 represents a hydrogen atom or a carboxyl group, _R2 represents a hydrogen atom, a methyl group, an ethyl group or a carboxymethyl group, and M represents a proton, an ammonium cation or an alkali metal cation),
The structural unit (B) is represented by the following general formula (2):
(In formula (2), _R3 represents a hydrogen atom, a hydroxyl group, or a carboxyl group.) It consists of the following structural units (A), (B) and (C):
The content of the structural unit (B) is 5 mol % to 30 mol % based on the total of the structural units (A) and (B);
the content of the structural unit (C) is 0.1 mol % to 10 mol % based on the total of the structural units (A) to (C);
A number average molecular weight Mn of 500 Daltons to 20,000 Daltons (Da) and
a polymer having a ratio Mw/Mn of number average molecular weight Mn to weight average molecular weight Mw of 1.00 to 2.00;
the polymer is water soluble and has an upper critical solution temperature (UCST);
a viscosity of a 50 wt % aqueous solution containing the polymer is 10 mPa s to 10,000 mPa s at 25°C;
It is hydrophobic at temperatures below UCST and hydrophilic at temperatures above UCST.
Temperature-responsive carboxylic acid-based polymers.
The structural unit (A) is represented by the following general formula (1):
(In formula (1), _R1 represents a hydrogen atom or a carboxyl group , _R2 represents a hydrogen atom, a methyl group, an ethyl group or a carboxymethyl group, and M represents a proton, an ammonium cation or an alkali metal cation),
The structural unit (B) is represented by the following general formula (2):
(In formula (2), R3 _represents a hydrogen atom, a hydroxyl group, or a carboxyl group.)
The structural unit (C) is represented by the following general formula (3):
(In formula (3), Q represents a structural unit derived from an acrylamide vinyl monomer.) The carboxylic acid polymer according to claim 1 or 2, wherein the number average molecular weight Mn is 500 to 5,000 daltons (Da). The carboxylic acid polymer according to claim 1, wherein the structural unit (A) is a structural unit derived from at least one carboxyl group-containing vinyl monomer selected from the group consisting of acrylic acid, methacrylic acid, ethacrylic acid, itaconic acid, and maleic acid, and the structural unit (B) is a structural unit derived from at least one aromatic vinyl monomer selected from the group consisting of styrene, vinylbenzoic acid, and hydroxystyrene. The carboxylic acid polymer according to claim 2, wherein the structural unit (A) is a structural unit derived from at least one vinyl monomer selected from the group consisting of acrylic acid, methacrylic acid, ethacrylic acid, itaconic acid, and maleic acid; the structural unit (B) is a structural unit derived from at least one vinyl monomer selected from the group consisting of styrene, vinylbenzoic acid, and hydroxystyrene; and the structural unit (C) is a structural unit derived from at least one acrylamide-based vinyl monomer selected from the group consisting of acrylamide, methacrylamide, 4-acroylmorpholine, and 4-methacryloylmorpholine. The carboxylic acid polymer according to any one of claims 1 to 5, wherein the UCST of a 20 wt% aqueous solution is 50°C to 100°C, that of a 30 wt% aqueous solution is 40°C to 90°C, that of a 40 wt% aqueous solution is 30°C to 70°C, and that of a 50 wt% aqueous solution is 10°C to 60°C. The carboxylic acid polymer according to any one of claims 1 to 6, having a 50 wt% aqueous solution viscosity of 10 mPa·s to 5,000 mPa·s at 40°C. The carboxylic acid polymer according to any one of claims 1 to 7, wherein the polymer has a structure in which the terminal end is a carboxyl group. A method for producing a temperature-responsive carboxylic acid polymer that is hydrophobic below UCST and hydrophilic at or above UCST, comprising polymerizing a monomer mixture containing a carboxyl group-containing vinyl monomer and an aromatic vinyl monomer in a hydrophilic solvent using a hydrophilic azo radical polymerization initiator, or a hydrophilic azo radical polymerization initiator and a RAFT agent or a hydrophilic chain transfer agent,
the ratio of the aromatic vinyl monomer to all monomers in the monomer mixture is 9.0 mol % to 20.0 mol %;
the total ratio of the hydrophilic azo radical polymerization initiator, the RAFT agent, and the hydrophilic chain transfer agent to all the monomers in the monomer mixture is 0.90 mol % to 13.0 mol %;
method the carboxyl group-containing vinyl monomer is one or more carboxyl group-containing vinyl monomers selected from the group consisting of acrylic acid, methacrylic acid, ethacrylic acid, itaconic acid, and maleic acid;
10. The method according to claim 9, wherein the aromatic vinyl monomer is one or more aromatic vinyl monomers selected from the group consisting of styrene, vinyl benzoic acid, and hydroxystyrene. The method according to claim 9 or 10, wherein the monomer mixture further contains an acrylamide vinyl monomer,
the ratio of the aromatic vinyl monomer to all the monomers in the monomer mixture is 10.0 mol% to 15.0 mol%, and the ratio of the acrylamide vinyl monomer is 0.1 mol% to 5.0 mol%;
method. The manufacturing method described in claim 11, wherein the acrylamide vinyl monomer is one or more acrylamide vinyl monomers selected from the group consisting of acrylamide, methacrylamide, 4-acroylmorpholine, and 4-methacryloylmorpholine. The manufacturing method described in any one of claims 9 to 12, wherein the hydrophilic azo-based radical polymerization initiator is a hydrophilic polymerization initiator having a carboxyl group, the RAFT agent is a RAFT agent having a carboxyl group, and the hydrophilic chain transfer agent is a hydrophilic chain transfer agent having a carboxyl group. The manufacturing method described in any one of claims 9 to 13, wherein the hydrophilic solvent is one or more hydrophilic solvents selected from the group consisting of methanol, ethanol, propanol, butanol, acetone, and water. An aqueous solution containing 1% to 70% by weight of the carboxylic acid polymer described in any one of claims 1 to 8, which undergoes phase separation below the UCST and becomes a homogeneous solution above the UCST. The aqueous solution according to claim 15, wherein the content of the carboxylic acid polymer according to any one of claims 1 to 8 is 20% by weight to 50% by weight. The aqueous solution according to claim 15 or 16, wherein the phase separation temperature is 40°C to 60°C. A work solution for a forward osmosis membrane water treatment system or an osmotic power generation system, comprising the carboxylic acid polymer according to any one of claims 1 to 8 as a solute.